Kinetic study of free radicals in the titanium (III) - hydrogen peroxide - alcohol reaction system

Detailed kinetic studies on Ti(III)--H₂O₂--alcohol reaction systems have supplied evidence for decay reactions of hydroxy alkyl radicals (·R-OH) derived from methanol, ethanol, 2-propanol, and t-butanol. As determined from dependencies of the limiting slopes of the first order plots at long times for [H₂O₂]₀- (greater than or equal to) 5[TiCl₃]₀, [H₂SO₄]₀= 0.01 -- 0.10, and [ROH]₀ (greater than or equal to) 20[TiCl₃]₀, the empirical rate law is d[·R-OH]/dt = k(subscript --d) values for different runs. Average k(subscript --d) values range from 1.6 x 10² M�¹sec�¹ for ·CH₂OH to 4.0 x 10² M�¹sec�¹ for (CH₃)₂COH. These values establish orders of reactivity towards hydrogen peroxide as (CH₃)₂COH > CH₃CHOH > CH₂OH for C� radicals and CH₂C(CH₃)₂OH > CH₂C(CH₃)OH > CH₂CH₂OH for C₂ radicals. A more complicated behavior prevails at short times; the decay rate increases non-linearly from first order. This can be ascribed at least in part to reaction of ·R-OH--H₂O₂ reaction. Without H₂SO₄ included in the system, such reactions are apparently minimized because in this case, decay of ·CH₂OH adheres well to first order. Furthermore, buildup of Ti(IV)-complexed radicals derived from "·OH" (S� and S₂) is concurrent with the decay of ·R-OH. For [TiCl₃]₀ = [H₂O₂]₀ = 0.01, second order behavior prevails, arising from predominance of the radical combination reaction. For ·CH₂OH, k (2nd order) = 8.3 x 10� M�¹sec�¹ and for (CH₃)₂COH, k (2nd order) = 1.6 x 10� M�¹sec�¹. ..